Head mounted image displays (e.g. helmets, goggles, and eyeglasses incorporating miniature displays) and other compact display systems which provide data in alphanumeric, video, or graphic form have applications in avionics, medicine, entertainment, and wearable computers, as well as numerous other fields. See for example U.S. Pat. Nos. 5,348,477, 5,281,960, 4,806,001 and 5,162,828. There are three principal types of prior art head mounted display systems: "see-through systems," in which the displayed electronic image is combined with the ambient imagery so that the user can see both images; "see-around systems" in which the displayed image occludes a part of the ambient imagery; and "full-immersion systems" in which the entire ambient image is blocked, so that the user sees only the electronically generated image. All three types of systems use various means, including lenses and the like, to project the image into the viewer's eyes.
The simplest systems are of the see-around type in which the electronic display is provided with one or more lenses and suspended in front of the user's eyes. A principal limitation of the device is that the display and optical system must be moved with respect to the head, or the head must be moved, to enable the user to see ambient imagery in the occluded field. A significant part of the occlusion results from the supporting structure and housing, as well as from the display itself. A second limitation of such devices is that the device is suspended from the head (or helmet, strap or other support borne by the head), so that the mass of apparatus adds an undesirable weight and/or torque to the head. A third limitation of the device is that position of the exit pupil of the optical system cannot be fixed accurately, meaning that the exit pupil of the optical system must be large enough to accommodate various motions of the device that occur during use.
Full-immersion systems have many of the same limitations as see-around systems. The head mounted system must be removed to view any ambient imagery. Typically, the systems comprise displays and lens systems similar to the see-around display, or comprise a display, lens system and reflective screen. These systems involve high weight, torque and volume.
See-through systems involve the most complex optical designs. Generally, the see-through system comprises a display, lens system, and viewing screen or combiner. All of the limitations of the see-around display are shared by the see-through display, except for the need to remove the head-mounted system to see ambient images. However, for this benefit, it is necessary to add further optical components and thus weight to the system.
All three of the above head mounted display types have the further limitation of requiring that the optical systems be mounted in goggles, helmets, strap-on bands, unusually bulky sunglasses frames having large visors and the like, rather than more conventional optical supports (such as the more simple support provided by conventional eyeglass frames). This limitation requires users to become accustomed to wearing such devices.
Another limitation of prior art displays is the need to provide illumination. For example, head mounted display systems using liquid crystal displays require lamps to illuminate the display. These lamps consume power and generate heat near the user's head and add to the overall volume and weight of the system.
A further and key limitation of the prior art eyeglass displays is the use of optical paths external to the eyeglasses system. For example, in U.S. Pat. No. 5,348,477, Welch describes a system comprising an image relay and a set of lenses and screens mounted external to an eyeglass frame and eyeglass lens. The use of a free space optical path, combiners and the like make miniaturization in a form approaching conventional eyeglasses quite difficult. Furness et al., in U.S. Pat. No. 5,162,828, have attempted to address this limitation with a see-through system based on a transparent screen, such as found in a goggle, with a display located at the top of the goggle or eyeglasses, and a mirror, which may be fixed or adjustable, located at the bottom of the transparency. This approach shows reduced complexity, but the system still requires at least one mirror positioned in an unconventional exposed location below or behind the transparency. The eyeglasses disclosed by Perera (U.S. Pat. No. 4,867,551 and 4,751,691) and Bettinger (U.S. Pat. No. 4,806,011) also require mirrors suspended from eyeglass frames. A particular limitation of free-space optical systems with mechanically suspended mirrors results from the fact that such appendages have a greater probability that the optical surface of the mirror will suffer damage during use, or will break off, or will damage the user's eye upon accidental impact. Additionally, because these systems include unusual optical appendages, they do not approach the ideal form of conventional eye wear.
There are many examples of displays mounted on the head, sunglasses, eyeglasses and the like. Display systems that make use of the user's existing eyeglasses or sunglasses, which may include prescriptive correction, are more simple in nature because they obviate the need for adding prescriptive correction to the display system. However, prior art displays mounted on eyewear or head-wear have the general limitation of obscuring a significant amount of the user's field of view.
For example, Perera in U.S. Pat. No. 4,867,551 describes a display to be mounted on eyeglasses, the limitation of which is the high degree of occlusion of the user's field of view beyond the display, and the use of non-axial optics which introduces distortion. A high degree of occlusion is also present in an eyeglass mounted display offered for sale by Albacomp Computers Corp. Other companies, such as Liquid Image or Virtual Vision, provide displays that are suspended by a cable, gooseneck fixture or other mechanical support in front of one or both of the user's eyes. Similarly, displays from Reflection Technology have been mounted on eyewear in order to provide a computer display in a mobile fashion, but this approach also highly limits the user's view of the surroundings.
Not only do such prior art displays obscure a large part of the user's visual field, but also the display obscures the user's face. The obscuration of the visual field is a limitation particularly in applications in which the user must be free to focus on the work at hand, without diverting attention to adjusting or removing the display.
Generally, head-mounted and helmet-mounted display systems are based on miniature displays having a diagonal dimension of 4 cm or less. The display systems that use such miniature displays must provide a lens near the eye for magnification, and to make possible comfortable viewing at near distances. The lens and any other associated optics that must be placed near the eye are termed the "eyepiece." Most prior art head-mounted systems also place the display (for example a miniature liquid crystal flat panel display) near the eye as well, which requires both a support fixture for the eyepiece, and a conduit for electrical cables to the display. These components (wires, liquid crystal display, and any other required circuits) are placed within an opaque housing near the eye. Consequently, such systems block a portion of the user's visual field, and also obscure the user's face. Obscuration of the face reduces the desirability of using the device in social occasions.
A helmet-mounted display is also known in which the image is provided by a glass fixture suspended in front of the user's face. The glass fixture is mounted to the helmet and contains an eyepiece near the eye. The glass fixture serves as a mechanical support for the eyepiece as well as an image conduit for the display which is mounted to the helmet. The limitation of this system, however, is its size and weight, and its configuration as a helmet.